Oral Presentation

Effects of grain growth on the extinction curve, the metal depletion, and the polarization in the interstellar medium

Author(s): Hiroyuki Hirashita (ASIAA), Nikolai Voshchinnikov (St. Petersburg University)

Presenter: Hiroyuki Hirashita (ASIAA)

Dust grains grow their sizes in the interstellar clouds (especially in molecular clouds) by accretion and coagulation. Here we model and test these processes by examining the consistency with the observed variation of the extinction curves in the Milky Way. We find that, if we simply use the parameters used in previous studies, the model fails to explain the flattening of the far-ultraviolet (far-UV) extinction curve for large R_V (flatness of the optical extinction curve) and the existence of a carbon bump even in flat extinction curves. This discrepancy is resolved by adopting a `tuned' model, in which coagulation of carbonaceous dust is less efficient (by a factor of 2) and that of silicate is more efficient with the coagulation threshold removed. The tuned model is also consistent with the relation between silicon depletion (indicator of accretion) and R_V. Based on this successful model, we examined the wavelength dependence of optical polarization. In this model, the wavelength of the maximum polarization (lambda_max) displaces to longer wavelengths and the polarization curve becomes wider (K becomes smaller). The tuned models can also explain the observed trend between K and lambda_max . It is significant that the evolutionary effect appears in the perpendicular direction to the K-lambda_max relation.